And maybe go with 4e saving throw (roll 10 or more on a flat d20) for ongoing effects? Yes, that could work. Would ST proficiency still apply? Like, if a class gave you prof in Dex saves, would you add your proficiency to your score? Maybe 1/2 prof, or only +2? Just to be sure to avoid having caster needing to hit a total Dex score of 26 to land a fireball, for example.
This isn't a bad idea, as basically the DC for the save would work like AC does in combat with the attacker always rolling. But I think there might be some problems with this in the mechanics. First, most characters are only proficient in two saves. So for a character without proficiency in DEX saves, but with a DEX 16, would have a big advantage doing it as you suggest. Also, how would proficiency help? If the same character had a proficiency bonus of +4, would their DEX save DC be 20 instead of 16?
I wouldn’t add proficiency bonus to target numbers. The math works out so that the target number is within about 1.5 of the average roll the character would get on a proficient save.
16-17: 10.5 avg + 3 mod +2 prof = 15.5
14-15: 10.5 avg + 2 mod +2 prof = 14.5
12-13: 10.5 avg + 1 mod +2 prof = 13.5
10-11: 10.5 avg + 0 mod +2 prof = 12.5
8-9: 10.5 avg -1 mod +2 prof = 11.5
With point-buy, scores of 18-19 and 20 are only possible at levels 4 and 8 at the earliest, with +3 prof bonus coming at level 5 and +4 at level 9.
18-19: 10.5 avg + 3 mod + 2 prof = 16.5
18-19: 10.5 avg + 4 mod + 2 prof = 17.5
20: 10.5 avg + 5 mod + 3 prof = 18.5
20: 10.5 avg + 5 mod + 4 prof = 19.5
So this system would make it a little harder to hit untrained and high-ability targets, and a little easier to hit trained and low-ability targets on average, but it comes pretty close in the middle ranges.
If you want save proficiencies to still matter under this system, I’d suggest adding them to rolls to end ongoing effects.
This system would also make it very difficult to affect certain monsters with spells that target certain scores. That would make choosing the right spell for the right enemy far more important, as it could make a difference of 10 or more in the DC. YMMV on if that is an acceptable change or not.